Plasmonic Nanoparticle Chains via a Morphological, Sphere-to-StringTransition

Youngjong Kang, Kris J. Erickson, and T. Andrew Taton*

Department of Chemistry, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455

Received July 27, 2005; E-mail: taton@chem.umn.edu

One-dimensional (1D) chains of regularly spaced, metal nano-particles exhibit unique, coherent optical properties when thedistance between particles is short enough for near-field couplingof their surface plasmon resonances.1 For example, theoretical2 andexperimental3 studies have shown that a 1D nanoparticle chain canserve as a “plasmon waveguide”, through which optical energiescan propagate even though the chain is much narrower than thediffraction limit. Surface lithography,4,5 solution-phase templatedassembly,6 and dipolar interactions7 have been used to fabricateplasmonically coupled nanoparticle chains, with the goal ofprecisely controlling particle shape and size, interparticle spacing,and chain geometry. Herein, we report that nanoparticles that havebeen encapsulated within cross-linked, spherical micellar shells canbe induced to assemble into structurally defined chains by triggeringa morphological, sphere-to-string transition in the micelle compo-nent (Scheme 1).

Previous research has shown that direct morphological transitionsbetween spherical and wormlike block-copolymer micelles can beinduced by varying solvent quality, ionic strength, or pH.8 Inaddition, under special circumstances, this transition can befrustrated to yield intermediate, necklace-like structures.9 We havefound that this same frustrated transition occurs when metalnanoparticles surrounded by a layer of cross-linked, amphiphilicpoly(styrene-block-acrylic acid) (“Au@PS-b-PAA”)10 are exposedto conditions that reduce interfacial curvature in the copolymer(Scheme 1). Au nanoparticles were encapsulated within PSm-b-PAA13 (m ) 100, 160, or 250), and 50% of the PAA carboxylateswere cross-linked, as previously described (Table 1).10,11Transmis-sion electron microscopy (TEM) images of these starting structuresshowed single, isolated nanoparticles surrounded by copolymershells of uniform thickness (Figure 1a). As aqueous suspensionsof these nanostructures (between 5 nM and 1 pM in particles) weregradually exposed to increasing concentrations of NaCl, CaCl2,acetic acid (AcOH), or 1-(3-dimethylamino)propyl)-3-ethylcarbo-diimide methiodide (EDC)sall of which induce a transition fromspherical to wormlike micelles for PS-b-PAA alone8sthe Au@PS-b-PAA nanoparticles were progressively assembled into longer andlonger, 1D chains. For example, as [EDC] was titrated from 0 to∼90 µM, an aqueous suspension of Au@PS250-b-PAA13 graduallyturned from reddish to paler, bluish violet in color, due toaggregation of the gold nanoparticles and coupling of surfaceplasmon excitations.12 TEM analysis of these aggregates (Figure1b,c) showed that particles were assembled into chains rather thanrandom structures, and that for [EDC]< 90 µM, more EDC led tolonger chains. As EDC was added beyond [EDC]≈ 90 µM, TEMimages and visual inspection of the solution showed that theaggregates had re-dissociated into individual encapsulated Auparticles. We attribute this behavior to inversion of charge at themicelle surface. In principle, micelle-bound, cationic EDC ligandsinitially balance the surface charge from carboxylate anions enoughto allow a reduction in micelle surface curvature and the sphere-

to-string morphological transition. However, too much added EDCligand results in cationic surfaces that once again exhibit sufficientintra- and interparticle repulsion to maintain isolated, sphericalparticles. This process was confirmed in situ by absorptionspectroscopy (Figure 1e,f), which showed a gradual red-shift inthe absorption maxima characteristic of surface plasmon couplingfor low to intermediate concentrations of EDC, followed by a blue-shift for higher concentrations. The sphere-to-string transition wasalso induced by increasing solution ionic strength to screen surfacecharge (NaCl), adding bridging ions (CaCl2), or neutralizingdissociated carboxylates (AcOH). In all cases, the transition couldbe reversed by neutralizing or diluting the additive. However, when

Figure 1. (a-c) TEM images of transition from spherical Au@PS250-b-PAA13 (4, Table 1) to nanoparticle chains with increasing concentrationsof EDC; [EDC] ) (a) 0 µM, (b) 55 µM, (c) 87 µM. (d) TEM images ofrepresentative nanoparticle chains prepared from nanostructures withdifferent shell thicknesses: (top)3; (bottom)2. (e) Absorption spectra ofnanoparticle chains made from4 at different concentrations of EDC. Thewavelength of maximum absorption (λmax) is marked on each spectrum. (f)Variation in λmax with [EDC].

Scheme 1

Table 1. Structural Characteristics of Nanoparticle Chains Formedfrom Au@PS-b-PAA

sample dAu (nm) shell composition initial tshell (nm) dedge-edge (nm)

1 12 ( 1 PS250-b-PAA13 14 ( 1 24( 22 32 ( 3 PS100-b-PAA13 10 ( 1 17( 23 32 ( 3 PS160-b-PAA13 28 ( 4 50( 64 32 ( 3 PS250-b-PAA13 21 ( 2 35( 65 52 ( 5 PS160-b-PAA13 16 ( 2 27( 3

Published on Web 09/17/2005

13800 9 J. AM. CHEM. SOC. 2005 , 127, 13800-13801 10.1021/ja055090s CCC: $30.25 © 2005 American Chemical Society

EDC was used to induce assembly, the resulting nanoparticle stringscould be locked into place by immediately adding aqueous diaminelinker. This yielded permanent structures that were not affected bysolution conditions.

The sphere-to-string transition yielded chains of regularly spacednanoparticles up to∼20 µm in length. By contrast, when hydro-phobic Au nanoparticles were mixed with free PS-b-PAA am-phiphile, and solvent conditions were changed such that cylindricalmicelles were assembled directly around the nanoparticles,13 TEMimages showed poorly structured combinations of micelles andparticles instead.14 This is consistent with previous efforts to directlysynthesize nanoparticles within wormlike micelles, which yieldedless control over interparticle spacing.15 Some longer particle stringswere branched, as is also observed for wormlike micelles.16

Branching was sensitive to the conditions of micelle assembly andTEM sample preparation, and optimizing both of these stepsimproved the yield of unbranched, straight structures observed byTEM. Overall, the degree of branching in all samples was muchless than that in fractal particle precipitates.17 In addition, theaverage three-center angleθ (Scheme 1) for stringed particles withonly two neighbors wasg150° for all samples.18 Most importantly,TEM images of the micelles showed that interparticle spacing wasdetermined exclusively by the thickness of the micellar shell (tshell)in the spherical starting materials. As shown in Table 1, this methodwas used to construct chains from different Au particle sizes,polymer shell compositions, and shell thicknesses. In all cases, thefinal edge-to-edge interparticle distance (dedge-edge) was∼0.85 timesthe sum of the two shell thicknesses. Standard deviations indedge-edge

were less than 20%, and deviations in interparticle center-to-centerdistances were less than 10%. We argue that this consistency isdue to the glassy nature of the PS copolymer block, which cannotdeform during the assembly process. Because both the particlediameter and shell thickness of the Au@PS-b-PAA starting materialcan be specifically determined,11 assembly yields 1D nanoparticlearrays with precise structural control.

Far-field, polarized microspectroscopy has been previously usedto analyze unidirectional near-field plasmon coupling in nanostruc-ture arrays,5,20and we chose to use this method to test the potentialof micelle-templated assemblies for use as 1D optical conduits.Nanoparticle chains were straightened by depositing a droplet ofwarm suspension onto a silicon wafer and allowing it to dry.Structural combing21 at the drying droplet edge led to straighteningof the particle chains, as verified by SEM imaging. One particularchain which could be reproducibly located in terms of surfacemarkers was chosen for analysis by SEM (Figure 2a), dark-fieldoptical microscopy (Figure 2a, inset), and polarized scattering (dark-field) microspectroscopy (Figure 2b). Scattering spectra of this

single chain with different polarizations of incident light showedsignificantly different intensities and wavelength maxima, indicatingdirectional surface plasmon coupling.5

In conclusion, we have demonstrated a facile method ofspontaneously assembling plasmonic Au nanoparticle chains whichtakes advantage of a morphological micelle transition. We arecurrently investigating the effects of particle size, composition, andspacing on directional plasmon coupling in more detail, with thegoal of constructing plasmon waveguides with tunable interparticlecoupling by solution-phase self-assembly. We propose that thismethod provides a simple avenue to investigate the unique opticalproperties of 1D nanoparticle arrays for future optoelectronic deviceapplications.22

Acknowledgment. This work was supported by the Alfred P.Sloan Foundation (Fellowship BR-4527 to T.A.T.) and the ACSPetroleum Research Fund (38303-G5).

Supporting Information Available: Detailed synthetic proceduresand experimental details for far-field polarization spectroscopy, TEM,and SEM images for other Au nanoparticle chains. This material isavailable free of charge via the Internet at http://pubs.acs.org.

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JA055090S

Figure 2. (a) SEM and dark-field optical microscopy (inset, scale barrepresents 2µm) images of a straight nanoparticle chain made from5. Thesample position in the optical image is rotated 25° clockwise relative tothe SEM image. (b) Scattering spectra of the chain, collected under obliqueillumination with light polarized parallel (E|) or perpendicular (E⊥) to thelong axis. Spectra were collected from the aperture-limited, circled area inthe inset of (a);λmax(E|) ) 621 nm,λmax(E⊥) ) 607 nm. The solid lines areLorentz fits of the spectral data.

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